Uniform corrosion conditions

The sensor was tested in an accelerated corrosion experiment by immersion in nitric acid. Most of the strain relaxation occurred within 40 s, and the balance within 200 s. The strain relaxation due to corrosion matched the measured residual strain within an average of 4.5 percent. A disadvantage of this technique is that the response to uniform corrosion conditions is nonlinear. The explanation is that the residual strains are not imiformly distributed, in a cross-sectional sense, across the metal shell. This technique is currently being developed to monitor atmospheric corrosivity on aircraft. 

Ail homogeneous metals without differences in potential between any points on their surfaces are subject to this type of general attack under some conditions. Uniform corrosion is usually characterized by a chemical or electrochemical attack over the entire exposed surface, Figure 4-423. Metal corrodes in an even... 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

Rp data are meaningful for general or uniform corrosion but less so for localized corrosion, including MIC. In addition, the use of the Stem-Geary theory where the corrosion rate is inversely proportional to Rp at potentials close to is valid for conditions controlled by electron transfer, but not for the diffusion-controlled systems frequently found in MC. 

There is no good substitute for an internal inspection. Assuming the hazardous chemical residues can be easily removed, the internal inspection is easily accomplished and relatively inexpensive. Internal inspections are essential to determine possible weakening of the vessel or any conditions that may develop into unwanted leaks, because very few vessels experience uniform corrosion. Typically, internal inspections are established with frequencies between semi-annually and once every ten years. The exact frequency is best determined by the corrosive nature of the chemical being processed or stored, including the effect of trace components and the past history of this equipment. 

As is well known, however, non-uniform corrosion under uniform or homogeneous solution conditions does occur so that die values of (ia)M wd/or (ic)Qean be larger at some sur -face sites than at others (21). These differences may arise from variations in composition and structure heterogeneities due to composition may supply areas which are predominately cathodic or anodic depending upon the valued of Eeq, iQ, ot and B of the different reacting systems at the particular sites. 

For materials like stainless steels, the mechanisms are quite different. Corrosion resistance in stainless steels is provided by a passive film that acts as a barrier between the alloy and the water. The passive film is a continuous, non-porous and insoluble film, which, if broken under normal conditions, is self-healing. Due to these characteristics, the uniform corrosion of stainless steels is usually very low and the major risk is pitting corrosion. The pitting corrosion risk of stainless steels is influenced not only ly the composition of the alloy and by water quality but also by service conditions, quality of the material and quality of the installation (fitting, soldering conditions, etc.). 

Both oxidation and reduction reactions that occur as corrosion proceeds can result in pH changes. The O2 reduction and hydrogen evolution reactions both either consume protons or produce hydroxyls. Therefore, under conditions in which there is a spatial separation of cathodic and anodic reactions, the pH at the cathodic sites will increase. Hydrolysis of metal cations will result in a decrease in pH at anodic sites. Even for uniform corrosion, with distributed and changing anodic and cathodic sites, the pH will tend... 

Industrial environments can be oxidizing or reducing in nature in terms of electrode potential that the alloy experiences, which is controlled by the cathodic reaction. Uniform corrosion occurs under reducing conditions and localized corrosion such as pitting and crevice corrosion occur under oxidizing conditions. SCC can occur in any potential range (50). 

These conditions will cause various deviations in the geometry and appearance of the attack compared with uniform corrosion, and it is convenient to classify corrosion just after the appearance of the corroded surface, as done by Fontana and Greene [7.1] (possibly with supplementary information about important service conditions). The advantage of such a classification is that a corrosion failure can be identified as a certain corrosion form by visual inspection, either by the naked eye or possibly by a magnifying glass or microscope. Sinee each form of corrosion has its characteristic... 

Parallel use of the Pourbaix diagram and overvoltage curves is sometimes useful when evaluating how typical and stable the uniform corrosion is under different conditions. Let us consider a case with iron or unalloyed steel in two different environments ... 

In severe (acidic) industrial atmospheres, a mixture of pitting and uniform corrosion may take place. The oxide film is partly dissolved under such conditions (compare the Pourbaix diagram). 

The characteristics of individual forms of corrosion are taken into consideration by providing appropriate corrosion specimens. Welded coupons having the surface quality of the material used later in practice are sufficient for determining uniform corrosion rates and acquiring general information on the type of local corrosion. Resistance to crevice corrosion can be determined by using specimens as described in ASTM G 78-83. Conditions of heat transfer can be simulated by using hot-wall/cool-wall specimens under temperature-controlled conditions. 

Corrosion can affect an entire surface of a metal or just local spots. Uniform corrosion of the complete surface usually only happens in acidic conditions [2]. This generally results in overall thinning and causes no major damage. On the other hand, a very detrimental form of corrosion is pitting. This type is found at a single location on the surface and creates a pit or cavity which is difficult to prevent and often hard to detect. It can result in structural failure (example a cracked pipe). 

Researchers have identified many different forms of corrosion. The rusting of automobile bodies is an example of uniform corrosion and is one of the most visible forms of corrosion. What conditions are necessary for this process to occur Another important form of corrosion is galvanic corrosion, which occurs only when two different metals contact each other in the presence of an appropriate electrolyte. What is so special about the contact of two different metals Other forms of corrosion tend to require specific conditions, yet many of these situations... 

The uniform corrosion rates for zinc are not greatly affected by the purity of zinc 98.5 and 99.99% zincs behave similarly in many conditions. This is especially true in open atmospheres, where sufficient oxygen is present to prevent polarization by hydrogen. Some alloying elements increase the corrosion resistance of zinc significantly. 

Immersion tests provide no information about reaction mechanisms and often they require relatively long exposure times. Electrochemical tests do not have these drawbacks and they are therefore widely used in practice. In the following electrochemical polarization methods are presented that provide information on the rate of uniform corrosion under conditions where the rate is controlled by charge-transfer. Other electrochemical test methods will be presented in subsequent chapters. 

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